As coating methods of car bodies, generally those for forming multilayer coating film by 3-coat-2-bake (3C2B) system comprising, after applying an electrocoating paint on the coating object, application of intermediate paint→curing by baking→application of water-based base coating paint→preheating (preliminary heating)→application of clear paint→curing by baking, have been widely adopted. Whereas, for energy-saving, attempts are made in recent years to omit the bake-curing step after application of intermediate paint and adopt 3-coat-1-bake (3C1B) system comprising, after applying an electrocoating paint on the coating object, application of water-based intermediate paint→preheating (preliminary hearing)→application of water-based base coating paint→preheating (preliminary heating)→application of clear paint→curing by baking (e.g., see JP 2002-282773A).
However, because the intermediate paint, base coating paint and clear paint are applied one on another in uncured condition in the above 3-coat-1-bake system, layer mixing is apt to take place between adjacent coating films, which occasionally impairs smoothness or distinctness of image of resulting coating film.
As a countermeasure to the trouble, JP-2004-275966A discloses a method for forming laminar coating film excelling in finished appearance and chipping resistance, by carrying out the bake-curing step in multistages of low temperature-heating stage and high temperature-heating stage, each under specific temperature-time conditions. This coating film-forming method, however, is subject to problems that sufficient smoothness and distinctness of image may not be obtained when water-based paints are used as the intermediate paint and base coating paint.